Cosmology deals with the large-scale structure and geometry of the universe, its history, how it has evolved. It's overall particle and chemical make-up and how that has evolved. Its age.
Nowadays we have both theoretical and observational cosmology. One of the main reasons for putting up the Hubble Space Telescope was to accurately determine the expansion rate. And other cosmo parameters. Measuring a largescale parameter, like overall curvature or expansion rate would be observational cosmology.